(1) Field of the Invention
The invention relates to a process for controlling the position of the focus of an X-ray tube applicable to the monitoring of X-ray equipment equipped with X-ray tubes and to the fitting of the latter in shields. The invention also relates to a control apparatus for performing this process.
(2) Description of the Prior Art
In per se known manner, an X-ray assembly comprises an X-ray tube equipped with a target, called an anode, as well as an electron emitter, called a cathode. The cathode emits electrons in a directional manner, in order to form an electron beam, said electron beam striking the target very locally in a very small area called the X-radiation focus. The anodes and cathodes are contained in an envelope, in which there is a vacuum which aids thermoelectronic emission. Together the envelope and the electrode form the X-ray tube.
To permit its normal use, the X-ray tube is contained in a shield which protects the user from high voltage and X-radiation, contains the high voltage insulating oil necessary for operation and fixes the tube relative to the X-ray outlet port.
Only part of the X-radiation emitted from the focus is used and this is generally shaped like a cone, whose apex is the focus and whose cross-section is defined by the opening of the shield outlet port.
An essential condition for obtaining the requisite geometrical qualities of the X-radiation beam consists of the centering of the focus relative to said outlet port. Thus, in general, the focus is centered relative to the outlet port, in such a way that the geometrical axis of the latter passes through the focus. This geometrical axis of the outlet port consequently constitutes the axis of the useful X-radiation beam.
In order to form the X-ray assembly, the shield equipped with the X-ray tube is combined with a device, such as a beam limiter or localizer. For this purpose, the geometrical shapes of the outlet port of the shield are such that they serve as a reference for the mechanical assembly of the equipped shield and e.g. the beam limiter. The alignment of the axis of the beam limiter opening on the axis of the X-radiation beam is greatly simplified as a result of the prior centering of the focus.
X-ray assembly designers measure the centering of the X-ray focus compared with mechanical references on the outlet port. Use is made of a X-radiation sensitive detector or receiver means, such as, e.g., a camera fixed to the outlet port and bearing on the reference faces of the latter. This camera also has elements making it possible to form the image of the focus on a film or screen on which, by means of wires or etchings, is materialized the theoretical axis of the outlet port. The offcentering of the image of the focus makes it possible to calculate the offcentering of the focus relative to the port axis, as a function of the geometrical characteristics of the camera.
This method suffers from the disadvantages that it leads to the blocking of the useful irradiation field during the measurement, makes it necessary to interpret results, which prevents real time measurements and offers a limited sensitivity, so that there is a lack of accuracy. Apart from the problem caused by these disadvantages in connection with the control of the position of the focus for the centering thereof, they prevent the use of said method for controlling the position of the focus during the operation of the X-ray equipment to which the X-ray tube belongs.
It is in fact very important to know possible variations of the spatial position of the focus because, in certain circumstances, they can have serious consequences with regards to the quality of an X-ray picture.